Modern ciphers

How many padding bits must be added to a message of 100 characters if
8bit
ASCII is used for encoding and the block cipher accepts blocks of 64
bits?
Suppose that we have a block cipher where n = 64. If there are 10 1’s in
the ciphertext, how many trialanderror
tests does Eve need to do to
recover the plaintext from the intercepted ciphertext in each of the
following cases?

How many padding bits must be added to a message of 100 characters if 8-bit ASCII is used for encoding and the block cipher accepts blocks of 64 bits?
1 MODERN BLOCK CIPHERS
Substitution or Transposition ?
To be resistant to exhaustive-search attack, a modern block cipher needs to be designed as a substitution cipher.
Suppose that we have a block cipher where n = 64. If there are 10 1’s in the ciphertext, how many trial-and-error tests does Eve need to do to recover the plaintext from the intercepted ciphertext in each of the following cases? a. The cipher is designed as a...

USE OF MODERN BLOCK CIPHERS
Symmetric-key encipherment can be done
using modern block ciphers. Modes of
operation have been devised to encipher text of
any size employing either DES or AES.
Error Propagation
A single bit error in transmission can create errors in
several in the corresponding block.

USE OF MODERN BLOCK CIPHERS
Symmetric-key encipherment can be done using modern block ciphers. Modes of operation have been devised to encipher text of any size employing either DES or AES.
1 USE OF MODERN BLOCK CIPHERS
Electronic codebook (ECB) mode
Error Propagation A single bit error in transmission can create errors in several in the corresponding block. However, the error does not have any effect on the other blocks.

TRAPDOOR ONE-WAY FUNCTIONThe main idea behind asymmetric-key cryptography is the concept of the trapdoor one-way function.
TrapdoorOne-WayFunction(TOWF)
One-WayFunction(OWF)
1.fiseasytocompute.
2. f −1 is difficult to compute.
3. Given y and a trapdoor, x can be computed easily.

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Lecture 3: Block ciphers and the data encryption standard. This chapter include objectives: To introduce the notion of a block cipher in the modern context, to talk about the infeasibility of ideal block ciphers, to introduce the notion of the feistel cipher structure, to go over DES and the data encryption standard.